Doctor blade coating is a coating method where a web, e.g. a web of paper, paper board or other cellulose-containing material, which contacts a support member, e.g. a roll, is supplied with a coating mixture in excess in a first step. The coating mixture may have the form of a more or less viscous liquid, usually referred to as coating paste. In this text, however, the term coating mixture is used. In the next step, excess of coating mixture is scraped off and the coating layer is evened by means of a so called coating blade. According to conventional technique the coating blade consists of a resilient steel blade. The Swedish patent No. 507 926, however, discloses a technique employing a blade which has a soft coating, which contacts the web. The present invention in the first place is intended to be used in connection with any of these two blade types.
When a paper web shall be coated by means of a blade only on one side according to conventional technique, wherein conventional steel blades as well as steel blades having a soft contact surface can be used, the coating usually is not accomplished all the way out to the side edges of the web, but about 10 to 20 mm wide edge zones are left uncoated at the two edges. The reason for this is that if the coating would be performed right to the side edges, coating mixture would be applied also on the edge surfaces, which are more or less uneven and often not quite straight. As a matter of fact, a more plentiful amount of coating mixture in this case would be applied on the edges than on the surface of the web intended to be coated. The coating mixture which is applied on the edge surfaces also, when only one surface is being coated, partly passes over to the opposite, uncoated side of the web, and since this side has contact with rolls as the web is transported in the coating machine before the coating mixture has dried to its so called point of immobilisation (i.e. to tack-free state), coating mixture will deposit on said rolls, which gives rise to problems concerning operation and quality.
The technique to avoid coating the web right out to the side edges, when only one side is being coated, implies that said problems are avoided. However, a drawback with that technique is that the web material is not fully utilized because uncoated edge zones must be cut off. Another drawback is that the edge zones are not supplied with the same amount of liquid as the coated region, which poses a risk of tensions in the edge zones and at worse ruptures in the edge regions.
Another problem is that the end portions of the coater blade is subjected to more severe wear, i.e. the portions which correspond to the border zones between the coated surface and the uncoated edge portions of the web. The reason for this is that the coating mixture, which normally consists of pigment and binding agents dispersed in water, is depleted of liquid in the above mentioned border zones. This is considered to depend on the fact that the web surface, which consists of fibre material, absorbs liquid and that there is a possibility for the liquid in the border zones to be absorbed outwards towards the dry edge portions. Therefore, more liquid is absorbed in the border zones than in the region of the coated surface inside of the edge, which leads to the formation of border zones containing dryer coating mixture and therefore more severe wear. The greater wear in the said zone in the course with time will result in a thicker deposition of coating mixture in the border zones, i.e. excess of coating mixture in those zones which do not get ample time to dry to the point of immobilisation of the coating mixture before it contacts rolls, which also gives rise to problems because of depositions on drying cylinders and guide rolls.
According to another method, both sides are coated simultaneously, as the web is guided between blades facing one another as is disclosed in the Swedish patent No. 507 926. According to that method, the web either can be guided vertically upwards or vertically downwards. The web normally is coated right out to the edges. Subsequent to the coating station the web is guided without being touched through the drying section so far that the layer of coating mixture, referred to as the paste layer by professional men, has dried to a tack-free dry content level.
Also in that case, more liquid in the form of coating mixture is absorbed by the web edges than by the web surfaces, which may cause that the edges have not reached a tack-free dry content level when the web is brought into contact with web guiding rolls or drying cylinders.